1. Field of the Invention
The present invention relates to surface emitting lasers and optical coherence tomography apparatuses equipped with such surface emitting lasers.
2. Description of the Related Art
A vertical cavity surface emitting laser (sometimes referred to as “VCSEL” hereinafter) is a laser device that emits a laser beam vertically relative to an in-plane direction of a substrate. This surface emitting laser forms a resonator in the vertical direction relative to the substrate by sandwiching an active layer from above and below with two reflectors and emits light in the vertical direction relative to the substrate.
In a known surface emitting laser, a laminate, which includes an active layer and a lower reflector, and an upper reflector are disposed facing each other with a gap section (sometimes referred to as “air gap”) interposed therebetween (see Japanese Patent Laid-Open No. 2004-296972). This surface emitting laser is capable of varying the laser oscillation wavelength by changing the distance of the air gap.
In the surface emitting laser disclosed in Japanese Patent Laid-Open No. 2004-296972, the antinode of a standing wave is located at the interface between the laminate and the air gap so that the threshold current is reduced.
As a result of a diligent examination performed by the present inventor, the present inventor discovered a problem in a surface emitting laser such as that disclosed in Japanese Patent Laid-Open No. 2004-296972. Specifically, with the configuration in which the antinode of a standing wave is located at the interface between the laminate and the air gap, that is, when the optical path length from the lower reflector to the aforementioned interface is an integral multiple of λ/2 (λ denoting a laser oscillation wavelength), the variable wavelength range of the oscillation wavelength is small. In view of this, the present invention provides a surface emitting laser with a wide variable wavelength range for the laser oscillation wavelength.